1. Field of the Invention
The present invention relates to electrophotographic system image forming apparatuses such as a copying machine and a printer in which an electrostatic latent image formed on an image bearing member is developed by toner to obtain an image.
2. Related Background Art
An image forming process by an image forming apparatus will be described with reference to FIG. 4.
First, by setting an original G with a surface to be copied facing downward onto an original stand 10, and pressing a copy button, copying is started. When a unit 9 integrally formed of an original irradiation lamp, short focus lens array and CCD sensor irradiates and scans the original, and the lighting scan light reflected by the original surface is formed into an image by the short focus lens array, and is incident upon the CCD sensor.
The CCD sensor is constituted of a light receiving section, transfer section and output section. A light signal is converted to an electric signal in the CCD light receiving section, and successively transferred to the output section in the transfer section in synchronization with a clock pulse, and the charge signal is converted to a voltage signal, and subjected to amplification and low impedance treatment in the output section to emit an output. The analog signal obtained in this manner is subjected to a known image processing, converted to a digital signal and transmitted to an image forming section.
In the image forming section, the above-described image signal is received, and an electrostatic latent image is formed as follows. A photosensitive drum 1 is rotatively driven centering on a central spindle at a predetermined peripheral speed, and uniformly charged to provide a positive or negative polarity by a charger 3 in the process of the rotation. The light of a solid laser element 102 (FIG. 5) turned on/off in response to the image signal and emitted to the uniformly charged surface is scanned by a rotary polygonal mirror 104 (FIG. 5) rotating at a high rate, and the electrostatic latent image is successively formed on the surface of the photosensitive drum 1.
FIG. 5 schematically shows the constitution of a laser scan section 100 for scanning the laser beam in the above-described device. When the laser beam is scanned by the laser scan section 100, the solid laser element 102 is flickered at a predetermined timing by a light emitting signal generating device 101 based on the inputted image signal. Subsequently, the laser beam radiated from the solid laser element 102 is converted to a substantially parallel light flux by a collimator lens system 103, further scanned in the direction of arrow C by the rotary polygonal mirror 104 rotating in the direction of arrow B, and formed into a spot-like image on a scanned surface 106 of a photosensitive drum or the like by the group of f.theta. lenses 105a, 105b, 105c. By the scanning of the laser beam, an exposure distribution is formed on the scanned surface 106 for one image scanning. Furthermore, when the scanned surface 106 is scrolled by a predetermined amount vertically to the scanning direction for every scanning, the exposure distribution is obtained on the scanned surface 106 in response to the image signal.
A developing process will next be described. Generally, developing methods are roughly classified into four types: a method comprising coating a sleeve with nonmagnetic toner with a blade or the like, or coating the sleeve with magnetic toner using a magnetic force, carrying the toner, and developing the image on the photosensitive drum in a non-contact state (mono-component non-contact development); a method of developing the image of the toner coated as described above in a contact state (mono-component contact development); a method comprising using a mixture of toner particles and a magnetic carrier as a developer, carrying the developer by the magnetic force, and developing the image on the photosensitive drum in the contact state (two-component contact development); and a method of developing the image of the above-described two-component developer in the non-contact state (two-component non-contact development). In respect of the high image quality and high stability of the image, the two-component contact development method is frequently used.
As shown in FIG. 3, a developing device 4 is provided with a developer container 16, the inside of the developer container 16 is divided into a development chamber (first chamber) R1 and an agitation chamber (second chamber) R2 by a partition wall 17, and replenishing toner (nonmagnetic toner) 18 is contained in a toner storage chamber R3. Additionally, the partition wall 17 is provided with a replenishing port (not shown), and the amount of the replenishing toner 18 corresponding to the amount of the consumed toner drops for replenishment in the agitation chamber R2 via the replenishing port.
On the other hand, a developer 19 is contained in the development chamber R1 and the agitation chamber R2. The developer 19 is a two-component developer containing the nonmagnetic toner and magnetic particles (carrier), and the mixture ratio is set so that about 4 to 10% by weight of the nonmagnetic toner is mixed. Here, the nonmagnetic toner has a volume average particle diameter of about 5 to 15 .mu.m. Moreover, the magnetic particles contain ferrite particles coated with a resin, or resin particles with magnetic materials dispersed therein, the weight average particle diameter is in a range of 25 to 60 .mu.m, and a volume resistance is in a range of 10.sup.6 to 10.sup.12 .OMEGA..multidot.cm. Furthermore, the permeability of the magnetic particles is in a range of 2.5 to 5.0.
The developer container 16 has an opening in the vicinity of the photosensitive drum 1, and a development sleeve 11 protrudes to the outside via the opening. The development sleeve 11 is rotatably incorporated in the developer container 16. The development sleeve 11 has an outer diameter of 32 mm, and a peripheral speed of 280 mm/sec, and rotates in a direction shown by an arrow in FIG. 3. The development sleeve 11 is disposed so that an interval from the photosensitive drum 1 is about 500 .mu.m. The development sleeve 11 is formed of a nonmagnetic material, and a magnet 12 is fixed inside as magnetic field generating means.
The magnet 12 has a development pole S1, a magnetic pole N3 positioned on the downstream, and magnetic poles N2, S2, N1 for carrying the developer 19. The magnet 12 is disposed in the development sleeve 11 so that the development pole S1 is substantially opposite to the photosensitive drum 1. The development pole S1 forms a magnetic field in the vicinity of the development section between the development sleeve 11 and the photosensitive drum 1, and a magnetic brush is formed by the magnetic field.
A blade 15 is disposed above the development sleeve 11 and at a predetermined interval from the development sleeve 11. An interval between the development sleeve 11 and the blade 15 is about 800 .mu.m. The blade 15 is fixed to the developer container 16. The blade 15 is formed of nonmagnetic materials such as aluminum and SUS316, and regulates the layer thickness of the developer 19 on the development sleeve 11. A carrying screw 13 is contained in the development chamber R1. The carrying screw 13 is rotated in a direction shown by an arrow of FIG. 3, and the developer 19 in the development chamber R1 is carried in the longitudinal direction of the development sleeve 11 by rotatively driving the carrying screw 13.
A carrying screw 14 is contained in the agitation chamber R2. The carrying screw 14 carries the toner along the longitudinal direction of the development sleeve 11 by its rotation. The toner freely drops into the agitation chamber R2 from a replenishing port 20 of the storage chamber R3.
The development sleeve 11 bears the developer in the vicinity of the magnetic pole N2, and the developer 19 is carried toward the development section with the rotation of the development sleeve 11. When the developer 19 reaches the vicinity of the development section, the magnetic particles of the developer 19 are connected by the magnetic force of the magnetic pole S1, and raised from the development sleeve 11 to form the magnetic brush of the developer 19.
For a developing system, a reversal developing system is used, and a direct-current voltage and alternating voltage are applied to the development sleeve 11 from a power source (not shown). In this conventional example, the direct-current voltage of -500V, an alternating voltage Vpp=2000V, and a rectangular wave Vf=2000 Hz are applied. Generally, when the alternating is applied, the development efficiency increases, and the image obtains a high grade, but fog is easily generated. Therefore, usually by producing a potential difference between the direct-current voltage applied to the developing device 4 and the surface potential of the photosensitive drum 1, the fog is prevented.
In this example, the fog removing potential is 150V which is a difference between the first uniformly charged potential of -650V and the direct current voltage of -500V applied to the development sleeve 11. On the other hand, a difference of 300V between the exposed and attenuated potential of -200V and the direct-current voltage of -500V applied to the development sleeve 11 corresponds to a contrast potential for attaching the toner to the photosensitive drum from the development sleeve.
The toner image formed in this manner on the photosensitive drum 1 is electrostatically transferred to a transfer material by a transfer charger 7 (see FIG. 1). Thereafter, the transfer material is electrostatically separated by a separating charger 8 (see FIG. 1), conveyed to a fixing device 6, and thermally fixed so that the image is outputted.
On the other hand, after the toner image is transferred, the surface of the photosensitive drum 1 is subjected to removal of adhering contaminants such as a transfer residual toner by a cleaner 5 (see FIG. 5), and repeatedly used for image formation. The above-described constitution is only an example. For example, instead of a corona charger, a fur brush, a magnetic brush, or a charging roller is used as a charger 2, a transfer roller is used as a transfer charger, and various systems are used. However, the image is basically formed in the above-described process.
In recent years, the miniaturization of the image forming apparatus has advanced. However, when only the above-described charging, exposure, developing, transfer, fixing and cleaning processes are miniaturized, the miniaturization of the entire apparatus is limited. Moreover, the above-described transfer residual toner is recovered and discarded by the cleaner 5, but it is preferable also from the respect of environment protection that such waste toner should not be generated. Therefore, the cleaner is removed, and a cleaner-less apparatus is proposed to perform the cleaning simultaneous with developing by the developing device.
The cleaning simultaneous with developing is a method of recovering a slight residual toner on the photosensitive drum after the transfer by a fog removing bias during developing on and after the next process. According to this method, since the transfer residual toner is recovered and used in the subsequent processes, the waste toner is eliminated, and maintenance troubles can be reduced. Moreover, a great space advantage is provided, and the apparatus can remarkably be reduced in size. Additionally, to enhance the cleaning efficiency in the simultaneous developing/cleaning system, for example, the use of a substantially spherical toner manufactured by a polymerization process and very superior in mold release properties is very effective.
However, the present inventors have found that the following problems are caused in the process of image formation on several tens of thousand of sheets in the simultaneous developing/cleaning system in which the apparatus can be reduced in size as described above, as compared with the system having the cleaner.
(1) The image density is gradually lowered particularly in a low humidity environment.
(2) The amount of flying toner increases particularly in a high humidity environment.
As a result of studies by the present inventors, it has been found that these phenomena are attributed to the inadequacies of the particle diameter of the toner developed on the photosensitive member, particle diameter of the residual toner on the photosensitive member, triboelectrification amount of these two toners with the magnetic carrier, and the like.
Specifically, for example, for the developing process, when the solid toner image is formed on the photosensitive member under the alternation electric field, and when the weight average diameter of the toner particles forming the toner image is larger than the weight average diameter of the toner particles in the developer container, during repetition of the image formation, the toner particles in the developer container shift toward the small diameter side. As a result, when the image is repeatedly formed, the developing property and image density are deteriorated as compared with the initial stage.
In the apparatus provided with the cleaner, developing bias conditions, and the like may be set so that the toner particle diameter on the photosensitive member becomes equal to the toner particle diameter in the developer container. However, since the simultaneous developing/cleaning system is constituted to recover the residual toner on the photosensitive member into the developer container (generally, since the toner passes through the transfer process, the toner particle diameter becomes smaller than that in the developer container, or an external addition ratio is lowered in many cases), the toner particles in the developer container more easily shift to the small diameter side as compared with those in the system provided with the cleaner.
In this case, when the triboelectrification amount of the residual toner on the photosensitive member with the magnetic carrier in the developer container is higher than that of the toner in the developer container or the replenishing toner container with the magnetic carrier, the electrification amount increases as the image formation advances. Additionally, the ratio of the small particle diameter toner increases as described above. Therefore, the image density is remarkably lowered particularly under the low humid environment. On the other hand, when the triboelectrification amount of the residual toner on the photosensitive member with the magnetic carrier in the developer container is lower than that of the toner in the developer container or the replenishing toner container with the magnetic carrier, the ratio of the toner particles with the small diameter and low triboelectrification amount increases in the developer container, and the toner flying amount increases particularly under the high humid environment.
Specifically, as described above, the studies of the present inventors have revealed that in the simultaneous developing/cleaning system, since the relation between the particle diameter of the toner for developing the electrostatic latent image on the photosensitive member and the particle diameter of the toner remaining on the photosensitive member and recovered into the developer container, and the relation in triboelectrification amount between these toners are not adequately set to maintain the initial state of the toner particle diameter distribution in the developer container, the above-described problems occur.